Berkeley Lab Generic Presentation

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Atomic hydrogen exposure of strained layer GaAs
photocathodes
M. Baylac, JLab baylac_at_jlab.org
P. Adderley, J. Brittian, J. Clark, A. Day, J.
Grames, J. Hansknecht, M. Poelker, P. Rutt, C.
Sinclair, M. Stutzman
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Polarized electrons

• Nuclear physics program requires an electron
  source with
• High efficiency
• High polarization
• Photoemission out of strained layer GaAs
  semiconductors

-100 kV
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Atomic hydrogen source
wafer
300C

• Semiconductor samples cleaned by exposure to
  atomic hydrogen
• H2 dissociation via RF inductive discharge
• Voltage can be applied on wafer to enhance/reduce
  effect of ions

15 cm
100 MHz
20 W
Mc.Alpine Schildknecht, Proceeding of IRE,
1959 (2099)
H2, or D2
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Polarization vs Hydrogen dose

• Significant depolarization
• Wavelength dependent
• Effect seen with and w/o ions

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Why this depolarization?

• Related to strain?
• Tested and ruled out
• Hydrogen trapped in material?
• Increased anneal cycle (12 h instead of
  2) no effect
• Variation of angle of incidence of light onto
  wafer?

Roughness measurements with high resolution
profilometry (Andy Wu _at_ Jlab ) Bare surface RMS
155 A Hydrogen exposed RMS 8500 A
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Conclusions

• Atomic hydrogen exposure necessary to obtain high
  QE with anodized samples
• Heavy dose depolarizes semiconductor
  significantly
• Depolarization with/without H ions, unexplained
  enhancement
• Excessive dose can reduce QE
• Surface analysis shows roughened surface which
  can explain depolarization (underway)
• Need prepare clean sample with minimal hydrogen
  exposure